Project 3:The overall goal of Project 3 is to determine the clinical significance of genetic variation in alpha2- adrenergic receptors (a2-ARs) in humans. The sympathetic nervous system, through alpha and beta adrenergic receptors, is a key regulator of cardiovascular response. Three a2AR subtypes - a2A, a2B, and a2C - are present in humans; studies in genetically modified animals have clarified the function of each subtype. The a2A-AR regulates central inhibition of sympathetic activity, and thus decreases norepinephrine release and blood pressure; the a2B-AR subtype mediates peripheral vasoconstriction and is required for salt-induced increases in blood pressure; the a2C-AR subtype regulates epinephrine concentrations. Recently, genetic variations were described in the human a2A, a2B, and a2C-AR genes, and shown to alter function in vitro. Little is known about the in vivo effects of variation in a2ARs, although some variants have been associated with increased risk of heart failure and myocardial infarction. We have resequenced the human a2A, a2B, and a2C-AR genes, and identified novel variants and defined haplotypes. However, the in vivo functional contribution of these to interindividual differences in sympathetic response, are not known. We will use an approach that has been highly effective in studies of.beta-AR genetic variations, of preselecting healthy individuals with the variants of interest, and studying them under carefully controlled conditions, to determine the relationship of genotype or haplotype with a well-defined phenotype. In Specific Aim 1 we will test the hypothesis that variation in the a2A-AR gene results i) in altered regulation of norepinephrine concentrations in response to physiologic sympathetic activation, and ii) in altered response to stimulation by systemic infusion of the agonist, dexmedetomidine. Specific Aim 2 will test the hypothesis that variation in the a2B-AR gene results in i) altered vascular desensitization in vivo, and ii) altered blood pressure response to salt. Specific Aim 3 will test the hypothesis that variation in the a2C-AR gene alters epinephrine concentrations in response to hypoglycemia. The translational studies proposed will define the contribution of a2-AR subtype genetic variants to physiological and pharmacological responses in vivo, and provide fundamental mechanistic information to guide future targeted studies in cardiovascular disease.